This invention generally relates to fluid catalytic cracking (FCC) applications. FCC is a catalytic hydrocarbon conversion process accomplished by contacting heavier hydrocarbons in a fluidized reaction zone with a catalytic particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke are deposited on the catalyst to provide coked or spent catalyst. Vaporous lighter products are separated from spent catalyst in a reactor vessel. Spent catalyst may be subjected to stripping over an inert gas such as steam to strip entrained hydrocarbonaceous gases from the spent catalyst. A high temperature regeneration with oxygen within a regeneration zone operation burns coke from the spent catalyst which may have been stripped. Various products may be produced from such a catalytic hydrocarbon conversion process, including a naphtha product and/or a light product such as propylene and/or ethylene.
Propylene is an important starting material in the petrochemical industry for the production of higher olefins, polypropylene and many other important products. Commercially there is a demand for FCC technology capable of producing high propylene yields from conventional feedstocks. The first step taken to increase FCC propylene yield is to add a medium pore zeolite catalyst, such as ZSM-5, to the catalyst blend. ZSM-5 catalyses cracking of gasoline range olefins to light olefins and will achieve 10-12 wt % propylene yield at standard FCC conditions. In order to increase propylene yield further, adding a product recycle to the reactor system or decreasing unit hydrocarbon partial pressure is necessary. Both methods significantly increase unit capital cost,
Decreasing hydrocarbon partial pressure increases FCC propylene yield by reducing hydrogen transfer between olefins and larger more stable molecules. Hydrogen transfer between olefins and larger molecules at adjacent catalyst active sites can reduce olefin yield in exchange for increased cyclic-olefin, saturates, and aromatic yields. By decreasing hydrocarbon partial pressure, the probability that molecules are adsorbed to adjacent active sites is reduced, therefore decreasing the chances of hydrogen transfer between molecules. The drawback to operating at low partial pressure is that actual volumetric flow rate at process conditions is increased for the same mass flow rate due to operating at lower unit pressure and increased steam rates; resulting in higher capital costs.
What is needed is a means to reduce hydrogen transfer between molecules on the FCC catalyst without decreasing hydrocarbon partial pressure. This invention proposes the addition of a hydrogen adsorption material into FCC catalyst that adsorbs hydrogen released from larger more stable molecules. By adsorbing hydrogen, the adsorption material will competitively inhibit hydrogen from being transferred from larger molecules to olefinic molecules at adjacent active sites on the catalyst. Such a material would help achieve the propylene yield benefit observed by decreasing hydrocarbon partial pressure without having to increase capital cost of the unit.